How do we track time once we leave Earth behind

Humankind is in the middle of a renaissance of space travel and exploration.

Since the last Apollo moon mission in 1972, government space programmes around the world have innovated little, reducing what was once an exhilarating race to space to simply circling Earth. That is now changing. Private companies like SpaceX, Virgin Galactic, XCOR and others have taken up, with a vengeance, the mission to go to space, creating transformative new rocket and space exploration technology in the process.

Elon Musk, the head of SpaceX, is committed to settling Mars within the next two decades. Achieving this goal is what Mr Musk calls “a fundamental bifurcation of the future of human civilization”.[1] For Mr Musk, moving human beings into exploring and settling space is central to our survival as a species. Travel to other planets is coming—and sooner than we might think.

So, how will we track time when some of us are living on Mars? Surprisingly, because Mars rotates at roughly the same speed as Earth, the Martian day (referred to as a “sol”) is 24 hours and 39 minutes long, a mere 2.7% longer than an Earth day. But the Martian year is 668 sols long, versus Earth’s 365-day year. When tracking time for its Mars Rover missions, NASA, the US space agency, relied on local Martian mean solar time: noon at any point on Mars is determined by when the sun is directly overhead. In this way, timekeeping on Mars is not unlike what humans have done on Earth since prehistory.

As we move farther out into space and away from our solar system, local mean solar time will not be an option. On any future spacecraft heading out beyond our solar system, time will have to be tracked using spacecraft clock time, the self-generated equivalent of local mean time. Clocks on spacecraft leaving Earth on this hypothetical future journey would likely initially be set to Coordinated Universal Time (UTC), as marked at Earth’s prime meridian near Greenwich, England.[2]

We say “initially set” because if humans are to move out beyond our solar system and still be alive when they arrive at their destination, they will need to travel at very high rates of speed. The faster they travel, the more time will become relative.

Albert Einstein theorised that time passes differently for any observer of time depending on the speed at which that observer himself or herself is moving.[3] Scientists have since proven Einstein’s theory, known as “time dilation”. Time dilation even applies to someone walking versus someone standing still, but at those speeds, the differences in how time passes for each person are far too small to be detected. For deep space travellers moving at much higher rates of speed, the real-world implications of time dilation could prove to be quite unnerving.

Let’s suppose that sometime in the future, humans figure out how to travel close to the speed of light (186,000 miles per second[4]). Now, imagine the first of a set of twins remains on Earth, while the second boards a spacecraft and spends two years flying into deep space and back at something close to the speed of light. Upon returning home, our space traveller discovers that her twin, who remained on Earth, is now not 2 years older, but 30 years older. Why? Because, as Einstein predicted, time moves more slowly for objects in motion than for objects that remain at rest.

If companies like Virgin Galactic and XCOR have their way, over the next decade, space travel will become as commonplace as air travel is now, with hops to the moon possibly counting as a weekend getaway. What is far more interesting to consider, however, is what will happen if and when humans begin to travel to other planets. Depending on how fast and how far humankind travels out into the universe, space flight will not simply change how we track time. It will change time itself.

Welcome to the Dawn of Commercial Space Flight

Comment

It’s stated that time move faster when they are away from gravitational body (i.e. away from earth) however, we say we age slower than those on earth, how does that work? Shouldn’t those on earth age slower since their time move slower?

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